mx28: fix SPL code to make USB booting work

This patch fixes booting i.MX28 CPUs via USB download.
In this mode the CPU's bootrom implements a USB HID device that
accepts a bootstream.

When downloading the bootstream via USB, first the SPL code is
received and executed. Then the u-boot image is received and
called.

The USB bootmode is interrupt driven.

This patch fixes two things:

1) The ARM's fast interrupt mode is disabled when the SPL code
has been run. So save and restore the CPSR register.

2) Save and restore c1 control register: the exception vector
location needs to be set back to bootrom space to make the USB
interrupts work again. The SPL code needs to change this
option for the ram size probing.

Signed-off-by: Matthias Fuchs <matthias.fuchs@esd.eu>
Acked-by: Marek Vasut <marek.vasut@gmail.com>